Recently, there is increasing interest and development of non-invasive or minimally invasive methods to kill tumor cells. In particular, thermal ablation is being investigated as an alternative and/or supplement to traditional methods of tumor destruction. Several methods have been developed and are being developed for various forms of cancer including, among others, cancers of the breast, prostate, lung, kidney, and liver. Methods of introducing localized heat include Radio Frequency Ablation (RFA), microwave therapy, extracorporeal or direct focused ultrasound, laser ablation, and other interstitial heat delivery methods including therapeutic ultrasound applicators. In particular, the use of high intensity focused ultrasound (HIFU) is currently being used as an approach for thermal therapeutic intervention for uterine fibroids and has been examined for possible uses in the treatment of liver, brain, prostate, and other cancerous lesions. The use of ultrasound is desirable because it allows the non-invasive treatment of deep tissues with little or no effect on overlying organs. Ultrasound therapy for tissue ablation works by sonicating a tissue of interest with high intensity ultrasound that is absorbed and converted into heat, thereby raising the temperature of the targeted tissues. As the temperature rises above 55 degree centigrade, coagulative necrosis of the tissues occurs resulting in immediate cell death. The transducers used in therapy can be outside the body or be inserted into the body e.g. through blood vessels, urethra, rectum etc.
Nowadays, magnetic resonance guided high intensity focused ultrasound (MR-HIFU) therapeutic system is commercially available to make the therapy procedure safe and efficient. In particular, MR-HIFU ablation is a promising technology in the field of oncologic intervention, e.g., ablation of benign tumors in the uterus, so-called uterine fibroids.
Currently, contrast-enhanced MR imaging is considered the ‘golden standard’ for assessing therapeutic response of MR-HIFU ablation, expressed as non-perfused volume (NPV), immediately after MR-HIFU therapy. However, contrast-enhanced MR imaging requires injection of GD-contrast agent (GD-DOTA). Although inject of contrast agent is relatively well tolerated, it will inevitably bring discomfortable feelings. Even worse, for patients with history of allergic and nephrotic dysfunction, reactions after injection of contrast agent may be severe and life-threatening. Due to intravenous injection of contrast agent, contrast-enhanced MR imaging can only be performed to assess the treatment after the HIFU therapy is completed, which means the assessment result cannot be used to improve the HIFU therapy in a real-time manner. Although EP2636368A1 provided a medical apparatus capable of modifying the treatment plan of the heating system using the magnetic resonance data, the modification is based on thermographic MR images to ensure that adequate heating is accomplished at the target zone while safeguarding that other critical anatomic structures are left intact. There is no teaching how to modify treatment plan of the HIFU therapy according to therapeutic response of HIFU ablation to improve the therapy efficacy.